The present invention relates to radio frequency (RF) power amplifiers and methods for minimizing distortion by employing adaptive control circuitry, particularly in feed forward power amplifiers, and the application of such control circuitry in wireless telecommunication systems, especially multi-carrier amplifiers.
Amplifiers are used to amplify an electrical signal in voltage, current or power. Amplifying devices are typically not perfectly linear over the whole of their operating ranges, and the nonlinearities in their transfer characteristics introduce distortion at the output of the amplifier.
Class "A" and "AB" amplifiers at best produce -30 to -35 dBc third order intermodulation (IM3) distortion unless they are operated significantly below maximum power. To do so to achieve a desired power level requires a very high power amplifier which is not always a viable solution. A well-known and established linearization technique is the use of feed-forward circuitry with the amplifier. However, the potential problem with standard feed-forward circuits is in the design of the distortion (error) amplifier and the fact that for a class-A or class-AB main amplifier, at least 25 to 30 dB of further cancellation is required to achieve -60 dBc IM3. The implication is that an amplitude error of less than 0.5 dB and a phase error of less than one degree must be achieved for proper operation of the system. Although the amplifier system can initially be set to meet these performance specifications, it is widely believed, as a result of component aging, environmental conditions, and other factors which introduce drift, such systems are dynamic and will inevitably fail to sustain required performance levels.
One potential solution is to create a feedback loop based on a dynamic vector modulator which can be added to the amplifier assembly with a feed-forward linearizer. The dynamic vector modulator might be controlled through a microprocessor or DSP-based controller. A high level block diagram of such a feed-forward amplifier with an adaptive controller is shown in FIG. 2.
U.S. Pat. No. 3,922,617, issued Nov. 25, 1975 to Denniston, discloses a feed-forward amplifier system as shown in FIG. 3 in this system, a sample of the input signal is subtractively combined with a sample of the output signal to produce a sample of the distortion products. The distortion sample is adjusted in phase and amplitude and subtractively combined with the device output to produce a distortion-reduced system output. First and second pilot signals, applied to the input of the feed-forward amplifier and the output of the main amplifier respectively, are detected in the sample of distortion product and the system output in order to produce control signals which adjust the phase and amplitude of the input signal and the distortion signal to provide an system which automatically compensates for uncontrolled variations caused by system components and operating environment.
The Denniston system requires a coherent detection network for the injected pilot signals to provide the correct control signal for phase and amplitude adjustment. Coherent detection adds inherent complexity to the system and makes implementation significantly difficult, especially for multi-carrier systems.
U.S. Pat. No. 4,412,185, issued Oct. 25, 1983 to Gerard, describes another feed-forward amplifier system as shown in FIG. 4. Referring to the numbered regions of the figure, the signals from 8 and 5 are subtracted at 7 to provide a distortion signal amplified and inverted at 10 which is combined at 11 with amplified signal 26. A reference signal 13 is injected at main amplifier 2 into the in-band frequency, such that it appears at the output terminal as if an amplified-induced distortion. Monitor circuit 14 monitors reference signal 13, present at output terminal 3, and modifies the characteristics of phase and amplitude equalizer 15, so as to remove injected reference signal 13 from amplified output signal at point 3.
As with the Denniston system, the reference (pilot) signal used is either a single tone, which is adjusted successively to a desired reference frequency, or employs a comb of frequencies like those typically generated by a comb generator. Where a single reference signal is employed, it must be monitored and employed to repeatedly adjust the appropriate frequency band of the equalizer for each successive reference frequency in order to perform a cancellation. Where a comb of frequencies is used, monitoring must be frequency selective, and therefore it must adjust and respond to each particular comb frequency while the appropriate band of the equalizer is adjusted.
While this patent teaches a system that attempts to achieve intermodulation product cancellation over a wide range of frequency bands, it nonetheless suffers from the shortcoming that several equalizer band adjustments must be performed before a desired degree of distortion cancellation is achieved. The tire required to perform these successive corrections adversely impacts system distortion cancellation and performance.
The prior art teaches the down-conversion of RF signals to DC, prior to sampling and digital processing, thus resulting in the introduction of spurious noise and so-called "DC offset." These imperfections, though not problematic for many applications, can seriously degrade system performance when introduced into the digital domain, especially in cases where the control of system components is sensitive to minor changes in sampled and processed RF signals.
What is needed is a distortion minimization system for feed-forward amplifiers, including multi-carrier feed-forward amplifiers, which rapidly and with precision, maintains a close dynamic balance between delay and amplification branches of both main and distortion (error) amplifier loops, thus maximizing distortion cancellation in a multitude of conditions. Furthermore, such systems must be easy to implement, economical, and not introduce undue complexity.